1. Field of the Invention
The present invention relates to an improvement in igniters for solid fuel rocket motors. The improved igniter has particular application in high performance full head-end web rocket motors that do not accommodate a conventional head-end or aft-end case mounted igniter.
2. Description of the Prior Art
The conventional igniter for large solid fuel rocket motors is a miniature rocket motor that is known in the art as a "pyrogen" igniter. Typically, a pyrogen igniter is mounted in the forward end of a motor through a hole in the propellant. In some high-performance rocket motors, however, a pyrogen igniter is impractical because of the full head-end web of the propellant in the forward end of the motor. The head-end web propellant grain design is advantageous in that it provides a higher mass fraction, minimizes nozzle throat size, which, in turn, maximizes the nozzle expansion ratio within the motor length envelope, and minimizes insulation weight.
An igniter for such high performance rocket motors must be mounted on a forward portion of the propellant, thereby to provide the ignition source of heat as close as possible to the surface to be ignited. When so mounted, however, upon ignition of and burning of the propellant, the support for the igniter erodes. This tends to cause the igniter to become detached from the propellant and presents a problem of possible resultant damage to the rocket motor nozzle or the propellant. In order to avoid such damage, the igniter must be substantially consumed before becoming detached from the propellant. Additionally, the igniter must perform its intended function of igniting the propellant before it is consumed.
An igniter having especial utility in igniting solid fuel rocket motors having full, head-end webs is disclosed in U.S. Pat. No. 4,498,292 which was granted on Feb. 12, 1985 to C. Max White, one of the joint inventors of the present invention, and which is assigned to the same assignee. The igniter of the White U.S. Pat. No. 4,498,292 is consumable, wafer-like in appearance, and includes a hat-shaped or shallow cup-shaped urethane housing or container. The housing comprises a flat disk attached to one end of a short cylinder, a flange being attached to the other end for attachment of the igniter directly to the rocket motor propellant. A wafer or disk of high energy solid propellant is attached to the inside of the housing. The disk of solid propellant has holes formed therein and has radial grooves in the surface facing the rocket motor propellant for directing gases from the burning propellant through holes or exhaust ports that are provided in the peripheral surface of the cylinder. A liner of EPDM, a terpolmmer elastomer made from ethylene-propylene diene monomer elastomer, on the inside surface of the cylinder has holes that are in alignment with the holes in the cylinder but which, normally, are smaller. An igniter initiator having a tubular urethane housing is positioned adjacent the solid propellant disk. Upon ignition of the solid propellant disk, as the internal pressure within the combustion chamber of the igniter becomes greater, the holes in the liner expand, releasing the hot gases in smoothly increasing volume to provide rapid ignition of the rocket motor propellant before the igniter is consumed by combustion or becomes detached from the rocket motor propellant due to erosion of the latter.
While a qualified consumable wafer igniter and having especial utility for igniting solid fuel rocket motors having full head-end webs, the igniter of the White U.S. Pat. No. 4,498,292 has been found to experience non-acoustic instability phenomena as illustrated in FIG. 5 of the drawings. This phenomena is undesirable and renders the igniter marginal or unacceptable for some applications.
Thus, there is a need and a demand for improvement in consumable wafer igniters to the end of eliminating the aforementioned non-acoustic instability phenomena. The present invention was devised to fill the technological gap that has existed in the art in this respect.